1. Field of the Invention
The invention relates to the field of bipolar transistors, particularly those used in conjunction with MOS transistors. More particularly, the invention relates to protecting the emitter-base junction against damaging reverse voltages.
2. Prior Art
The reverse biasing of the emitter-base junction in bipolar transistors which can result in current gain degradation, is a well recognized problem, particularly where the bipolar transistors are incorporated into MOS integrated circuits. The problem is aggravated when the speed of the bipolar transistor is raised by thinning-out the base region. The increased doping in the base region lowers the reverse breakdown potential of the emitter-base junction. This problem is discussed in "Bipolar Transistor Design for Low Process-Temperature 0.5 .mu.m Bi-CMOS", IEDM-237, by M. Norishima, Y. Niitsu, G. Sasaki, H. Iwai and K. Maeguchi; and in, "Hot-Carrier Reliability of Bipolar Transistors", IEEE 28th Annual Proceedings, Reliability Physics 1990, by Burnett and Hu beginning at page 164.
One solution to this breakdown problem uses a reversed bias polysilicon diode across the emitter-base junction. While this is a satisfactory solution from an electrical standpoint, it often requires the use of an extra mask. In other instances, a bipolar junction transistor is used although this solution typically requires more substrate area. Additionally, MOS devices have been used to solve this problem. In some cases, the MOS devices do not provide sufficient protection.
As will be seen, the present invention uses an additional bipolar transistor formed in the same well as the bipolar transistor sought to be protected.